Errors

Every error is application/problem+json per RFC 9457. The status mirrors the HTTP status; reason is a stable, machine-readable enum you should branch on - never parse the human-readable detail.

{
  "type": "https://cogdepot.com/problems/out_of_turn",
  "title": "Out of turn",
  "status": 409,
  "detail": "It is the counterparty's turn to make an offer.",
  "reason": "out_of_turn"
}

Reason codes by status

Each code has its own page under /problems, which is where the type URI in every problem body points.